In recent years, in the event of a large-scale disaster across a wide geographical area, research for quickly and accurately confirming the range of space (confined space) isolated from the surrounding areas is being advanced. The quick and accurate confirmation of confined spaces will be useful in making decisions for rescue operations and relief supplies. As a conventional technology for confirming a confined space, for instance, there is a technology of dividing a geographical space into a plurality of small areas (grids), counting the number of persons remaining in the grids based on mobile object data including location information which is sent from a portable terminal as appropriate, and extracting the grids in which there is no change in the number of persons as the confined space.
Note that technologies of confirming the safety of users during a disaster by using portable terminals have been previously disclosed in various documents. For instance, PTL 1 discloses a technology of promptly confirming the safety of users based on location information sent from the portable terminals and information indicating whether they are located in the disaster area. Furthermore, PTL 2 discloses a technology of estimating that users, who did not move before and after the occurrence of a disaster, were affected by the disaster based on location information before and after the occurrence of the disaster sent from the portable terminals.